U.S. Pat. No. 5,122,232 discloses a steam spray tube and a process for controlling the amount of steam dispensed by the steam spray tube. The steam spray tube here is arranged beneath a web of material passing through a calender where at least one roller has a highly polished surface. The steam spray tube dispenses steam through its nozzle arrangement and the steam then condenses in the air and is precipitated in the form of a cloud or mist on the web of paper passing by. The resulting increase in moisture content of the paper web has the effect that the paper web can be smoothed better in the downstream roller gap and/or has a higher gloss. The gloss and/or smoothness of the paper web are measured at the end of the calender and the measured values are sent to a control device that controls the valves of the steam spray tube. The valves are designed as digital valves so only a limited precision is possible with regard to the amount of steam dispensed. In order to improve the precision, the pressure to all the steam tubes is readjusted according to given mathematical methods.
One problem with such a moistening process is that a rather thick film of air adheres to the web of material, moving along with it and preventing or at least greatly interfering with the penetration of the steam or the cloud formed by the steam on the web of material. This effect is greater as the speed of travel of the web of material increases. At the same time a web of material traveling at a high speed requires considerably more steam applied per unit of time in order to maintain the same moisture load as a web of material traveling at a slower speed. Furthermore increasing the steam pressure in order to increase the outlet velocity of the steam is not without hazards. At a high steam pressure and a resulting higher steam outlet velocity from the nozzle arrangement, the steam may entrain droplets of water that have formed somewhere in the inlet line or in the steam spray tube itself and may throw them at a high velocity against the web of material where these droplets of water act like projectiles that can perforate the web of material and thus greatly reduce its quality.
Therefore, one object of the present invention is to ensure adequate moistening of the web of material at high processing velocities.